A photoinitiator is scientifically a compound transforming the physical energy of light into suitable chemical energy in the form of reactive species. Upon absorption of light with a specific wavelength, it undergoes a photoreaction, producing reactive species, and these reactive species may be free radicals for a free radical polymerization system or cations for a cationic polymerization system.
The photochemical polymerization of unsaturated monomers and pre-polymers is a well known methodology and has wide industrial applications from radiation coating to radiation printing. The most reactive photosensitive systems for radical polymerization reactions are usually the cleavable photoinitiators, and particularly these derived from acetophenone. In these photosensitive systems, the photoinitiators are excited into their singlet state and covert, through intersystem crossing, to their triplet state. These excited states can undergo a direct formation of two radicals. Then, one or both of these radicals can initiate the polymerization reaction through an addition reaction onto the monomer double bond leading to the first monomeric radical.
For example, U.S. Pat. No. 4,582,862 discloses specifically an acetophenone-type photoinitiator, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propane-1-one (MMMP), which has been widely used as a photoinitiator nowadays, under the commercial name of Irgacure 907 from Ciba, Basel. Upon excitation of UV-light, The MMMP molecule undergoes a bond cleavage and generates two free radicals as shown in Scheme-1.
After polymerization and post-curing, these free radicals are quenched and mostly remained in polymeric matrix as low molecular weight residues. In the case of MMMP, these residue molecules include 4-methylthiobenzaldehyde and N-isopropylmorpholine as exampled in Scheme-1. The yielded residue molecules, in particular, the sulphur-containing fragment, 4-methylthiobenzaldehyde, not only generates a strong irritated odour, but also causes respiratory tract irritation if inhaled and skin irritation if absorbed through skin.
In order to reduce the irritation and odour, researchers have investigated many MMMP derivatives without sulphur content, such as these disclosed in U.S. Pat. No. 5,145,885 and U.S. Pat. No. 6,048,667. Even though some of these derivatives, did exhibit less odour than MMMP, when being used as a replacement for MMMP in ink formations, they found limited applications due to their thermal and mechanic un-stability. By incorporating selected stabilizers into the ink formulation may improve the ink stability and improve its shelf-life, but the addition of polymerization stabilizers often retards the photo-curing speed, eventually reduce the photo-initiation efficiency of such photoinitiators.
In view of the limitation of small molecular weight photoinitiators, macro-type photo-initiators have drawn increasingly interest especially for UV-curable formulations. Improvements have been claimed in terms of low-odour and non-yellowing properties.
For instance, Hitoshi Yamato et al in U.S. Pat. No. 6,673,850 assigned to Ciba Specialty Chemicals Corporation disclosed a class of marcophotoinitiators comprising chain transfer groups and these macrophotoinitiators were claimed to be polymerized to yield block copolymers. Shaun Lawrence Herlihy in U.S. Pat. No. 7,612,122 assigned to Sun Chemical Corporation described a group of dendrimer-type marcophotoinitiators with poly-hydroxy as the linking core.
Furthermore, Chingfan Chris Chiu in US Patent Application (US2004/0068110) disclosed a group of polyester-linked marcophotoinitiators. The disclosed polyester-linked marcophotoinitiators increased the length of the sulfur terminal by chemically bonding to a polyester chain, thus offered the advantage of low volatility and low odor after curing. However, like most of polymeric photoinitiators disclosed in the prior art, this group of polyester-linked macrophotoinitiators also suffer from the lower photo-curing speed than the traditional small molecular weight photoinitiators, though they consist of the same cleavable function moiety, morpholinoketone as that in the traditional small molecular weight photoinitiators such as Igacure 907.
It is now the objective of this invention to provide a photoinitiator which posses the comparable photo-curing speed to that of the analogues of small molecular weight photoinitiators but also posses the advantage of low odor/low extractability of these macrophotoinitiators.
It is another objective of this invention to teach the procedure to manufacture the invented photoinitiators.
Yet, it is still another objective of this invention to describe the use of the invented photoinitiators in radiation curing formulations.